Desulphurizing reaction mechanism in ladle furnace was analyzed. Effecfion of slag compostion on sulfur distribution ratio and variation of sulfur content were studied. Optimal slag content range was presented： w（CaO）：52 %-57 %,w（A1203）：35 %-40 %, w（SiO2）≤6 %, w（FeO＋ MnO）≤%. Through improving operation, w（S） in ladle furnace was reduced to≤0. 004 %. This can needs of low-sulfur steel and ultra-low-sulfur steel.